The present invention relates generally as indicated to pipe-to-soil potential difference surveys, and more particularly to a method and apparatus for making closely spaced pipe-to-soil potential surveys.
In controlling corrosion of pipelines or other elongate structures buried in the ground, an electrical potential is applied to the pipeline to reduce the potential difference to a minimum negative potential difference between the pipe and adjacent soil or water. To determine where the voltage should be applied and to assure that sufficient voltage is applied to the pipeline, electrical measurements are made to determine the pipe-to-soil difference along the length of the pipeline. The electrical potential is applied by conventional cathodic protection devices.
Such electrical measurement requires a contact to the pipe, a suitable voltmeter or potentiometer, a means of contacting the ground and connecting wires. A copper-copper sulfate (Cu-CuSO.sub.4) cell is an industry standard for providing the necessary contact with the ground.
Measurements are commonly made on a yearly interval basis at test lead stations. Test leads are wires connected to the underground pipe brought above ground and permanently mounted in a device to protect the wire and allow easy accessibility. Test leads are installed at locations along the pipe, usually spaced one to two miles apart.
In addition to this type of survey, which provides pipe-to-soil potential measurements at test lead locations only, a continuous, over the pipeline, closely-spaced survey is conducted occasionally to determine the pipe-to-soil potential difference measurements at intervals of, for example 10 to 25 feet along the length of the pipe.
Heretofore, a reel of relatively heavy insulated wire has been connected to the pipe at a reference location, preferably a test lead. The insulated wire would then be dragged from the reel across the ground along the route of the pipe and the copper-copper sulfate half cell would be placed directly over the pipe at intervals of 10 to 25 feet or whatever spacing desired. Distance from the test lead would be determined by chaining, or by measuring with a wire measuring device mounted at the reel at the reference location. The voltage measurement between the wire (connected to the pipe via the reel and test station) and the copper-copper sulfate half cell would then be made with a suitable voltmeter or potentiometer at the spaced intervals. Radio contact would be maintained between the man at the reel location, and the men at the end dragging the wire. This was necessary for distance determination and recording of the readings observed at the voltmeter located at the half cell location. When the next test lead would be reached (usually within 8,000 feet), the half cell would be disconnected, and the wire would be reeled back (usually by electric power) and transported to the next test lead location along the pipeline surveyed.
This system, however, has a number of disadvantages, namely the wire and reel and power re-winding mechanism are heavy and require truck or van transport. The reel itself may weigh on the order of 300 lbs. At most of the surveys are done over farm land, crops, etc., the use of a vehicle is generally prohibited, and the pipeline owner usually specifies that only walking over the right of way be allowed. Furthermore, the effort required to drag the wire is considerable often requiring several men, especially when trying manually to pull a mile of wire across uneven terrain. Moreover, the resultant wear on the wire and frequent breakage, plus electric reel maintenance, further add to the cost of this system.